As is well known in the art, the quality of a telephone call is frequently degraded by the terminals themselves and by the communications network equipment.
The deterioration generally results from one or more parameters such as echo, transmission time (or delay), distortion, signal loss, and signal compression. It is more particularly sensitive in the case of Voice Over IP (VoIP) telephone calls via the Internet, where telephone signals are sent in the form of packets after analog-to-digital conversion, coding, and compression.
Various solutions have been proposed in an attempt to limit the effects of one or more of the above parameters. Thus, echo may be controlled using echo controllers or suppressors conforming to the ITU-T G.131 standard. Transmission times and packet loss rates may be reduced by monitoring voice data traffic at the network level. Telephone terminals may also be adapted to allow users to vary the volume of the received sound, in particular if they have a hearing problem or the received sound level is very low.
Those solutions undoubtedly reduce the deterioration induced by the parameter concerned, but in some circumstances they may exacerbate the deterioration induced by some other parameter. Thus adjusting the receive sound level has negative effects on stability loops and on echo in particular.
Improving the signal-to-noise ratio by operating on the telephone signals before they are sent has also been proposed. Thus the methods described in documents EP 1 104 925 A1 and US 2002/0156623 A1 propose subtracting a noise function, or noise, from the voice signals to be sent just before they are coded. The method described in document WO 02/101722 proposes to substitute comfort noise for noise superimposed on the voice.
Operating on the original (or native) signals, the above three methods modify their nature, to the extent that the receiving user is unable to discern the sound environment of the sending user. What is more, those methods increase the number of processing operations that the sending telephone terminal has to perform, which consumes CPU resources that could be used for other tasks.
What is more, the prior art solutions take no account of recently developed methods of VOIP signal processing and in particular voice activity detection (VAD) mechanisms.
An object of the invention is to improve on the above situation, in particular in noisy environments, no prior art solution being entirely satisfactory.